Automatic volume control



Sept. 3, 1935. v G. JOBST ET AL 2,013,310

7 AUTOMATIC VOLUME CONTROL Filed Sept. 19, 1932 2 Sheets-Sheet 1 (MT/70L M1736! 01/7707 (O/V7770! mus;

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m w POTENTIAL INVENTORS GUNTH ER .JOBST ATTORNEY G. JOBST El" AL AUTOMAI I C VOLUME C ONTROL Filed Sept. 19, 1932 2 Sheets-Sheet 2 l I I I I 32 2a 24 20 /5 12 INVENTORS V GUNTHER JOBST ATTORNEY Patented Sept. 3, 1935 TATES PATENT OFFICE AUTOMATIC VOLUME CONTROL Application September 19, 1932, Serial No. 633,730 In Germany October 24, 1931 7 Claims.

The present invention relates to circuit arrangements to insure automatic volume control for fading elimination.

, For the purpose of eliminating fading it is known that the varying intensities of the incoming waves may be equalized, or compensated, by using radio frequency amplifier tubes so that the amplifier gain is made a function of the grid biasing potential, the grid biasing potential be- ]0 ing regulated in accordance with the intensity of the incoming waves. An automatically regulated circuit scheme of this sort provides for, and necessitates, the supply of a biasing potential to a radio frequency amplifier tube which is derived from. the rectified incoming energy. In the circuit arrangements heretofore used the grid circuit of the radio frequency amplifier tube usually contained an ohmic resistance which is traversed by the plate currents of a rectifier tube; but no rules whatever were observed in the combination of the rectifier and the radio frequency amplifier tubes as regards their rectifier and amplifier characteristics. Indeed, it is for this reason and the reason hereinbefore indicated that in the methods known in the prior art the fading effects are merely lessened, but no perfect compensating thereof is obtained.

The present invention, on the contrary, discloses ways and means whereby the dependence of the reproducer current upon the amplitude of the carrier wave in the ultimate effect disappears entirely with the result that the volume of reception depends only on the degree of modulation of. the transmitter. This result is attained by that in impressing or supplying the volume control potentials, special attention is given tothe proper choice of the amplifier characteristic of the tubes and the same, for instance, is employed in combination with a rectifier tube possessing a special rectifying characteristic adapted to the characteristics of the amplifier tube.

What isiof importance and value in this connection is that the amplifier tube should be of a type involving small plate reaction (i. e. screen 5 grid tubes) since in this type the gain is not affected by the alternating current potentials arising in the plate circuit. Besides screen grid tubes also other types will be found suited as long as theirinternal resistance is, high in con- 50" trastwiththe outside resistance.

In the drawings, Fig. 1 shows a form of the invention, Fig. 2a shows the characteristic of rectifier A,

Fig. 2h graphically shows-the operation of the invention in Fig. 1,

Fig. 3 shows the characteristic of tube Hz,

Fig. 4 shows a modification of Fig. 1.

The conditions underlying the invention may be explained mathematically approximately in the following way: 5

The output alternating current voltage of a radio frequency screen grid tube across a plate impedance R, in view of the fact that the internal resistance of the tube is high compared with the outside resistance, is given by the expression 10 S=S(Eg) Hence, there is V=RS(E )A(1+a sin Nt) sin Hi.

The aim is to render the output alternating potential V independent of the incoming amplitude A by appropriate selection of Eg as a function of A. Feeding an alternating potential to a rectifier, by the provision of suitable circuit means (parallel capacitor, series inductance coil, etc.) the said rectifier will insure' a rectified potential 35 'Q=F(A, a) J impressed upon a resistance contained in the plate circuit of the rectifier. By suitable choice of thecircuit means conditions can be made so that Q becomes only a function of the incom- 40 ing amplitude A in that the amplitudes of the audio frequency modulation are perfectly compensated. Q therefore varies only at the rate of the slow fading variations of the incoming amplitude. Hence, Q can be represented in this Way:

Impressing this quantity in the form of a biasing potential upon the radio frequency tube so that there becomes the demand. fcr'independence of the output 211- ternating potential of the carrier-wave amplitudes is fulfillable by suitable selection of the relations between slope (mutual conductance) S and the regulating potential Q derived from the rectifier; indeed, all that is necessary to that end is that S(Ego+F(A) A CODSt.

Upon this presupposition the alternating potential delivered from the radio frequency amplifier tube is independent of the amplitude of the carrier Wave. Fluctuations in intensity or volume of the incoming wave-train by variations of power of the transmitter, or arising out of fading effects, will not manifest themselves at all, while dependence of output currents upon the degree of modulation is perfectly preserved.

The requirement laid down in this invention, namely that the regulative potential supplementarily supplied to the control electrode of the radio frequency amplifier should so depend upon the amplitude of the incoming waves that the resultant mutual conductance of the screen grid tube acting as an amplifier becomes inversely proportional to the incoming amplitude, may be fulfilled in various Ways. One Way would consist in using a rectifier tube having such a rectifying characteristic that the rectified voltage derived across a resistance included in the plate circuit thereof when impressed in the form of an additional regulating potential upon the control electrode of the amplifier tube insures the desired conditions.

But it is also feasible to rectify the incoming energy in a rectifier of any desired kind, and to cause the rectified current to flow through such an amplitude-dependent resistance (valve or the like), and to thereupon derive across a resistance a suitable regulating potential for the radio frequency amplifier tube which, when applied to the control electrode, changes the mutual conductance of the tube inversely proportionally to the incoming amplitude of the radio frequency waves.

The above situation shall be illustrated by the aid of another example:

The incoming wave-train shall be represented A(l+a sin Nt) sin Ht(1) where A is the amplitude of the carrier-wave, a is the degree of modulation, N is the modulating frequency, and H is the radio frequency. The oscillations picked up by the grounded aerial A (see Fig. 1) are fed to two amplifier and rectifier channels I and 2. Associated with channel 2 containing the radio frequency amplifier tube H2 and the rectifier tubeAz is the loudspeaker L.

The radio frequency amplifier tube is of the screen grid type I-Iz having a characteristic of the.

In other words, S1 is equal to S for Eg= -1.

In the rectifying channel I is provided a linearly acting radio frequencyamplifier tube. H1 Which amplifies the input oscillations p-fold. Hence,

the radio frequency amplitudeimpressed uponv the plate rectifier A1 is represented by the expression I 11A(1+a sin Nt) sin Ht(H) The rectifier which is conditioned by the characteristic of the radio frequency amplifier tube H2 and is to furnish the galvanic mean of the radio frequency current, is to have a characteristic of the kind schematically illustrated in Fig. 2a. Across a resistance W shunted by a sufiiciently high condenser C it is possible to tap a direct current potential Q which will correspond to the value (Q. P. A.) where Q is another proportionality factor. In this scheme the condenser is chosen so large that it will equalize (or smooth) as much as possible the pulsations of the audio frequency modulation. The direct current potential Q resulting from the input potential is represented in Fig. 2b. The direct current potential is additionally impressed for regulating purposes upon the input electrodes of the amplifier tube H2. The latter is also supplied with the radio frequency energy,

A(1+a sin Ni) sin Ht either in whole or part. Its output potential is represented by SRA(1-|-a sin Ni) Sin Ht i. e. proportional to the mutual impedance and the input amplitude. Since as stated before, mutual conductance S is inversely proportional to the grid potential Eg, and since the grid bias potential consists of the direct current potential resulting from the rectifier N1 Q=qP there follows for the output potential of H2 this value:

RS V pq (1+a sin Nt) sin Ht.

Hence, the radio frequency potential available at the output end of the radio frequency stage H2 is entirely and absolutely independent of the amplitude of the carrier wave. The amplitude of the wave-train is here assumed to be so small that it is admittedly feasible to figure with the equations of linear amplification, in other words, the curvature in the characteristic is disregarded. The radio frequency potential thus obtained is thereupon further amplified and rectified and fed to an electro-acoustic device. The volume or intensity of the reproduction of the same will then be linearly dependent only upon the degree of modulation 0. in addition to being a function of the apparatus constants.

Fig. 4 shows a circuit arrangement in which the output current of the rectifier A1 is not proportional to the amplitude of the incoming waves. The output currents of the rectifier A1 therefore are fed to a thermionic tube E which represents an amplitude-dependent resistance in such a way that the current flowing in the plate circuit of the tube E occasions a potential across the resistance W which is proportional to the incoming volume or amplitude.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of our invention as set forth in the appended claims.

What is claimed is:

1. In a receiver for modulated carrier waves, the combination with a signal frequency amplifier tube whose plate current-grid voltage characteristic is substantially exponential and a demodulator, of a linear rectifier network for rectifying amplified signal voltages to produce a direct current voltage for automatically regulating the gain of said amplifier, said amplifier characteristic and rectifier linear characteristic being so related as to render the mutual conductance of the amplifier inversely proportional to the received carrier amplitude, and means conductively connecting the output of said network to the input electrodes of said amplifier tube.

2. In a receiver for modulated carrier waves, the combination with a signal frequency screen grid amplifier tube whose plate current-grid voltage characteristic is substantially exponential and a demodulator, of a linear rectifier network for rectifying amplified signal voltages to produce a direct current voltage for automatically regulating'the gain of said amplifiensaid amplifier characteristic and rectifier linear characteristic being so related as to render the mutual conductance of the amplifier inversely proportional to the received carrier amplitude, and means conductively connecting the output of said network to the input electrodes of said amplifier tube.

3. In a receiver for modulated carrier waves, the combination with a signal frequency amplifier tube whose plate current-grid voltage characteristic is substantially exponential and a demodulator, of a linear rectifier network for rectifying amplified signal voltages to produce a direct current voltage of such magnitude that the mutual conductance of the tube is inversely proportional at any instant to the amplitude of the received waves for automatically regulating the gain of said amplifier, and means conductively connecting the output of said network to the input electrodes of said amplifier tube.

4. In a receiver for modulated carrier waves, the combination with a signal frequency amplifier tube whose plate current-grid voltage characteristic has a remote negative cut-off, and a demodulator, of a linear rectifier network, including a non-linear rectifier stage and a linear amplifier for the output of said rectifier stage, for rectifying amplified signal voltages to produce a direct current voltage for automatically regulating the gain of said amplifier, said amplifier characteristic and rectifier linear characteristic being so related as to render the mutual conductance of the amplifier inversely proportional to the received carrier amplitude, and means conductively connecting the output of said network to the input electrodes of said amplifier tube.

5. In combination, a radio frequency amplifier tube of the type having a substantially exponential relation between plate current and control grid voltage, a demodulator network of the type having a substantially linear relation between direct current output and radio frequency input above a value of radio frequency input voltage, means for impressing radio frequency signal energy upon said network, the characteristics of said amplifier and demodulator network being so related as to render the mutual conductance of the amplifier inversely proportional to the amplitude of received signals, and means for impressing upon the input electrodes of said amplifier tube a direct current potential derived from said network.

6. In combination, a radio frequency amplifier tube of the type having a substantially exponential relation between plate current and control grid voltage, a demodulator network of the type having a substantially linear relation between direct current output and radio frequency input above a value of radio frequency input voltage, means for impressing radio frequency signal energy upon said network, and means for impressing upon the input electrodes of said amplifier tube a direct current potential derived from said network such that the mutual conductance of said amplifier tube is inversely proportional to the amplitude of said signal energy.

7. In combination, a radio frequency amplifier tube of the type having a substantially exponential relation between plate current and control grid voltage, a demodulator network of the type having a substantially linear relation between direct current output and radio frequency input above a value of radio frequency input voltage, means for impressing radio frequency signal energy upon said network, and means for impressing upon the input electrodes of said amplifier tube a direct current potential derived from said network, said direct current potential being the sole direct current potential existing between the said input electrodes.

GUNTHER JOBST. WALDEMAR WEHNERT. 

